Acidity, acid strength (pKa), pH and their interrelationship are described. The roles played by acids in foods either as endogenous constituents, added ingredients or generated in situ as products of bacterial fermentation are discussed along with the moderation of their effects by dilution due to food moisture and endogenous buffering capacity. The inhibition of growth and survival of microorganisms by acids is discussed along with the ways in which this can be used to control pathogens in foods. Some approaches to predicting the inhibition of pathogens by acidity show how safety can be designed into new food products and critical limits derived for critical control points in the manufacture of existing products. The limitations of acidity as a control measure are illustrated with some examples of outbreaks of foodborne illness in which they have been implicated.